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Abstract

This paper investigates the influence of the stress level and hygrothermal conditions on the creep/recovery behaviour of three high-grade composites made of GreenPoxy and flax and hemp fibres. The results show that the levels of instantaneous, time-delayed and residual strains increase with the applied load and the severity of the environment. The time-delayed strains of the materials are higher in the creep phase than in the recovery phase. A stiffening effect is also observed during the recovery phase. The post-creep viscoelastic properties are then identified using an anisotropic viscoelastic law from the recovery function. The relaxation time function is independent of the stress level and the environmental conditions. The viscous parameter varies with the stress level and increases substantially with severe environmental conditions. The dependence of the creep/recovery behaviour on the stress level is due to the dependence of the stiffening phenomenon and irreversible mechanisms rather than to the viscoelastic behaviour.
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... Among plant fibres, flax (Linum Usitatissimum L.) and hemp (Cannabis Sativa) fibres are now the two most produced bast fibres in Europe (Bourmaud et al. 2018). For their properties, such as important environmental advantages, good specific mechanical properties and often available at a viable cost (Bourmaud et al. 2018), these fibres have emerged as an alternative to synthetic fibres, and the use of plant fibre composites (PFCs) has become a market reality (Mohanty et al. 2018;CLEC 2018;Sala et al. 2021). Despite a lot of similarities (cell wall, thicknesses and numbers of layers/sub-layers, biochemical composition, cellulose microfibril angle, MFA), these both bast fibres generally exhibit differences in their tensile properties and their global tensile behaviour. ...
... Flax fibres generally have slightly better tensile properties than hemp fibres, especially in terms of tensile strength and stiffness; on the other hand, they reach a smaller tensile strain at failure than hemp (Del Masto et al. 2019). If a lot of works are conducted at the scale of fibres to explain these differences (Bourmaud et al. 2018;Del Masto et al. 2019;Baley et al. 2020;Réquilé et al. 2021), few papers are dedicated to the development of hemp continuous fabrics used as reinforcement of composite samples (Corbin et al. 2020a, b;Corbin et al. 2021;Sala et al. 2021). This deficit for hemp could be assigned to technological barriers, such as fibre separation and the alignment of fibres throughout the transformation process and consequently the unavailability of these products on an industrial scale . ...
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... Many studies have already shown that the moisture absorption in natural fiber reinforced composites implies a decrease in strength and rigidity (Al-Maharma and Al-Huniti 2019; Chaudhary, Kumar Bajpai, and Maheshwari 2020;Malloum, El Mahi, and Idriss 2019;Maslinda et al. 2017;Muñoz and García-Manrique 2015). Some of them have studied the effect of the water temperature on the absorption behavior (Sala et al. 2021;Scida et al. 2013). Nevertheless, to the best of authors' knowledge, there are no published results concerning the damage mechanisms due to tensile loading after hygrothermal aging for hemp/Greenpoxy and hemp/Elium composites. ...
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... The variations are not significant considering the scattering in measurements for each specimen type, with values between 2.5% and 4.5%. The mean values of both volume fraction and void contents are in agreement with results found in literature[29,32,33]. The fibre volume fraction is in the upper range of available data with a relatively low scattering of the results and the void content in the lower range, attesting the good reliability of the process even in presence of an insert. ...
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Plant fibers are being increasingly explored for their use in engineering polymers and composites, and many works have described their properties, especially for flax and hemp fibers. Nevertheless, the availability of plant fibers varies according to the geographical location on the planet. This study presents the first work on the mechanical properties of a tropical fiber extracted from the bast of Cola lepidota (CL) plant. After a debarking step, CL fibers were extracted manually by wet-retting. The tensile properties are first identified experimentally at the fibers scale, and the analysis of the results shows the great influence of the cross-section parameters (diameter, intrinsic porosities) on these properties. Tensile properties of CL fibers are also predicted by the impregnated fiber bundle test (IFBT). At this scale of bundles, a hackling step, which reduces shives and contributes to the parallelization of the fibers within bundles, improves tensile properties predicted by IFBT. The comparison with the properties of plant fibers given in the literature shows that CL fibers have tensile properties in the same range as kenaf, flax or hemp fibers.
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